Brittle fracture test pieces

FIGURE 17.8 Fracture behavior of two cylindrical cheese samples subjected to uniaxial compression. Sample (1) concerns a hard and fairly brittle cheese, (2) a semihard green cheese that is quite extensible, (a) Stress a versus Hencky strain eH, resulting upon deformation at a strain rate W of 1CT2 s 1 the dotted lines indicate the strain at fracture fr. (b) Values of Sfr obtained at various W. (c) Fracture mode as seen in cross sections through the test pieces at the moment of maximum stress for (1) it is a vertical cross section, for (2) a horizontal one. 

Materials may break in shear or in tension the former occurs in short (e.g., brittle), the latter in long (e.g., rubbery) materials. Another material property is notch sensitivity. In a test piece that is put under tension, notches can be applied, and in several materials the ensuing stress concentration greatly lowers the overall stress needed for fracture propagation. The notch sensitivity is smaller if the material contains defects of the size of a small notch, or if the bonds between structural elements are much stronger in the direction of the applied stress than in a perpendicular direction, as in many fibrous systems. 

Fracture toughness and tensile properties are summarized in Table 1 below. The ductile IPNs showed substantial drawing and yielding in the impacted test>pieces compared with either smooth or sometimes locally crazed fracture surfaces for the more brittle polymers. 

The slow growth of cracks in poly(methyl methacrylate) is an ideal application of linear elastic fracture mechanics to the failure of brittle polymers. Cracks grow in a very well-controlled manner when stable test pieces such as the double-torsion specimen are used. In this case the crack will grow steadily at a constant speed if the ends of the specimen are displaced at a constant rate. The values of Kc or % at which a crack propagates depends upon both the crack velocity and the temperature of testing, another result of the rate- and temperature-dependence of the mechanical properties of polymers. This behaviour is demonstrated clearly... 

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